The invention relates to a mobile device with an independent battery and a radiofrequency communication interface. More particularly, the invention relates to a piece of equipment with a small battery, such as for example an access card.
A communicating card device will be used as a concrete example of the embodiment of the invention. However, any other mobile device, which has an independent power supply and a radiofrequency communication interface, will be capable of implementing the invention described below.
The state of the art for a communicating card devices makes it possible, among other things, for a user to be authenticated by a remote computer system, such as for example a personal computer, a mobile telephone, a personal digital assistance or a device with which the user needs to interact, in a limited perimeter.
That authentication system comprises a mobile device that is carried by the user and is made up of the following:                A smart card that contains the user's secrets in a secure manner,        An electronic card holder with an interface for an ISO7816 smart card and a radiofrequency interface, e.g. of the Bluetooth type.        
Such equipment makes it possible to relay data between the smart card and the remote computer system. In this example, it is assumed that the remote computer system has a Bluetooth radiofrequency interface.
One of the key criteria of a mobile device with an independent power supply with a radiofrequency interface is its independence. The radiofrequency interfaces available (Wi-Fi, Bluetooth etc.) use techniques that are designed to increase the independence of such devices. However, the radiofrequency interface is not completely shut down and thus consumes energy regardless of its state. But there are states in which the radiofrequency interface can be put out of operation, in order to increase the life of the power supply.
Besides, the density of wireless equipment, for example in office environments, can help shorten the life of the power supply. That is because even when a mobile device is not in an active communication session, it interacts with the environment by listening so as to detect any request for communication. Such listening also consumes power, and that power consumption is not negligible.